For the detection of changes in speed on drive shafts, the prior art teaches the use, inter alia, of acceleration sensors which operate in accordance with the Ferraris principle or rotary field principle. Such sensors are based on the induction principle and are classed with induction measuring elements.
In accordance with the Ferraris principle, a disk, for example made for aluminum, is used as a conductor which is rotatably mounted and moves in a magnetic field. The magnetic field may also be designated as the main field, to be generated either by a permanent magnet or by an operating coil. The design of such a measuring head surrounding the Ferraris disk will be discussed in more detail below. Movement of the Ferraris disk, for example by coupling to a drive axle, induces currents, in particular eddy currents, in the Ferraris disk which can be evaluated by a detector coil and provide a variable, proportional to the acceleration of the Ferraris disk.
In practice, the solutions for evaluating such an acceleration sensor using the Ferraris principle in accordance with the prior art exhibit substantial drawbacks. For example, the sensitivity of the sensor drops sharply in the case of a high rotational speed of the Ferraris disk. The illustration according to FIG. 1 shows a diagram in this respect in which the output signal Vα of such an angular acceleration sensor according to the prior art is plotted as a function of rotational speed ω. From this it can be seen that the output signal of the sensor drops by 3 dB as early as at a rotational speed range of approximately 3000 to 3500 rpm. This characteristic response to the rotational speed of DC Ferraris sensors can be ascribed first to dissipation in the disk, which leads to heating of the disk, and second, to the eddy-current field, which acts in a compensating fashion on the applied DC magnetic field at a relatively high rotational speed.
Based on an extrapolation of the prior art, it would seem possible to avoid this undesired effect by using either the design of the sensor or electronic linearization, for example, to amplify the amplitude of the sensor signal as a function of the working point, to linearize the sensitivity. A drop of 3 dB in the useful signal can be displaced by higher rotational speeds by the design of the sensor, for example by skillful selection of the material of the disk, the applied magnetic field, and of the gap between head and disk. But these design choices result in a loss of sensitivity.
Linearizing in an electronic way could also be performed by post-amplification of the acceleration signal as a function of rotational speed. However, this requires knowledge of the sensitivity characteristic relative to rotational speed, and of the rotational speed itself. Moreover, interference such as noise is also amplified, and this leads to a smaller signal-to-noise ratio at higher rotational speeds.